As far as prior art is concerned, from DE 196 44 360 there is known a colour spraying booth for powder coating in which the excess lacquer particles are conveyed into a suction channel situated at the centre of the booth floor, and to this end there are provided two transverse airstreams in the horizontal direction. The transverse airstreams are produced by blowing air into the booth through horizontal slots in the side walls of the colour spraying booth. On each side wall of the colour spraying booth there is also provided a nozzle, constituted by a sheet metal part, by means of which the airstream produced by means of a ventilator is introduced into the interior of the colour spraying booth. When the cleaning of the booth floor is performed in this manner, however, it is associated with the drawback that the sheet metal parts attached to the outside of the sides considerably increase the width of the booth. A further disadvantage consists of the fact that the longitudinal sides of the booth have to be provided with slots for the sheet metal parts. If it does not prove possible to suck up a sufficient quantity of secondary air from outside via these openings and blow it into the booth, the openings will also have to be sealed after the building in of the sheet metal parts in order to avoid powder being lost from the booth through them.
A further possible way of designing a booth for powder spray coating with a cleaning device is known from the German patent specification DE 100 28 553. The powder spray coating booth is provided with a suction channel arrangement at the lower end of the booth interior for sucking air and excess powder out of the interior of the booth. The suction channel arrangement is designed in such a manner that that a non-uniform distribution of the suction airstream is brought into being at the lower end of the booth interior along the transport path of the object. The manner in which this is obtained is that the far greater part of the exhaust air is made to flow out of the booth space along the transport path of the object and through suction passages that are arranged in the vicinity of the passages that permit the object to be moved into and out of the booth space and therefore at the two ends of the booth. To this end the suction channel present in the booth is covered by means of two displaceable channel covering parts. Furthermore, inside the booth there are provided compressed air distribution lines, so that the excess powder is blown in the direction of the suction channel. But this construction form has the disadvantage that moving parts are present in the booth, which enhances the breakdown liability of the plant. Furthermore, the two displaceable channel covering parts and the slope of the booth floor make it difficult to walk in the booth.